This invention relates to an apparatus for the decontamination of radioactive metallic waste generated as in a nuclear power plant and required to be decontaminated for safe disposal.
Generally, radioactive metallic waste generated as in a nuclear power plant is put to permanent storage within the plant's premises and thereby prevented from producing adverse effects upon the environment.
When this practice of permanent storage is continued, however, the total amount of storage of such radioactive metallic waste will increase, eventually making it difficult to find a space for further storage. Pipes and other similar objects which are contaminated by radioactivity in the nuclear power plant have large dimensions and cannot be readily reduced in size by any of the existing treatments. These objects, therefore, are put in storage only with difficulty.
Studies are now under way in search of a method which is capable of decontaminating such radioactive metallic waste and lowering its level of radioactivity to the level of radioactivity existing in nature, namely the background level of radioactivity, and thereby enabling the metallic waste to be handled as safely as ordinary industrial waste.
The radioactive metallic waste, however, has contaminants adhered to its surface. Mere removal of contaminants loosely piled up on the surface hardly suffices for ample decontamination. Sufficient decontamination necessitates dissolution of surface layers of metallic matrices which are covered with adhered deposited contaminants.
Prior art methods of decontaminating such radioactive metallic waste include, a method of electrolytic decontamination which accomplishes the decontamination by electrochemically dissolving the surface layers of the metallic matrices and a method of chemical decontamination which chemically dissolves the surface layers of metallic matrices by use of a decontaminating agent.
The method of electrolytic decontamination is advantageous in that it effects decontamination at a high speed. It nevertheless entails the disadvantage that it cannot be applied effectively to metallic matrices of complicated shape because it requires the surface of the electrode to be opposed to the surface of the metallic matrix under treatment.
The method of chemical decontamination enjoys high adaptability to metallic matrices of complicate shape. It has the disadvantage, however, that the decontamination proceeds at a slow speed and the decontaminating agent is irreversibly consumed and consequently gives rise to a large volume of spent decontaminating agent which constitutes secondary waste.
Moreover, the decontaminating agents used in the apparatus for chemical decontamination and the apparatus for electrolytic decontamination are destined to be completely exhausted because of deterioration of performance and accumulation of radioactivity and inevitably give birth to secondary waste in large volumes.
U.S. Pat. No. 4,217,192, for example, discloses a method and system for the decontamination of metallic objects radioactively contaminated in the nuclear industry by means of chemical etching.
This chemical etching resorts to the effects of oxidation and reduction obtained by a procedure which comprises immersing, in an electrolytic solution formed of a nitric acid solution containing trivalent cerium ions and tetravalent cerium ions and held in one container, a given contaminated metallic object in conjunction with a pair of electrodes intended for regeneration of the electrolytic solution, allowing the metallic object to stand therein until it is relieved of radioactivity, and removing the decontaminated metallic object from the electrolytic solutiuon.
Since this method is operated batchwise and not continuously, it has the disadvantage that the operation itself entails extra time and labor each time one round of operation is replaced by the next one and the period of exposure of operators to radiation is long. Since this method uses a concentrated nitric acid solution at elevated temperatures, it has the possibility that the solution will emit nitric acid gas and nitric acid mist and impair the working environment. Further, this method has the disadvantage that the residue of etching produced within the electrolytic solution will be suspended in the form of slurry to hasten wear of the electrolytic solution, deprive the electrolytic solution of its function, and even shorten its service life.
In the disclosure, no measure for overcoming these disadvantages is not suggested at all.